Feldspathic clast populations in polymict ureilites: Stalking the missing basalts from the ureilite parent body

摘要

Polymict ureilites DaG 164/165, DaG 319, DaG 665, and EET 83309 are regolith breccias composed mainly of monomict ureilite-like material, but containing similar to2 vol% of feldspathic components. We characterized 171 feldspathic clasts in these meteorites in terms of texture, mineralogy, and mineral compositions. Based on this characterization we identified three populations of clasts, each of which appears to represent a common igneous (generally basaltic) lithology and whose mafic minerals show a normal igneous fractionation trend of near-constant Fe/Mn ratio over a range of Fe/Mg ratios that extend to much higher values than those in monomict ureilites. The melts represented by these populations are unlikely to be impact melts, because the ubiquitous presence of carbon in polymict ureilites (the regolith of the ureilite parent body) implies that impact melts would have crystallized under conditions of carbon redox control and therefore have highly magnesian mafic mineral compositions with constant Mn/Mg ratio. Therefore, these melts appear to be indigenous products of igneous differentiation on the ureilite parent body (UPB), complementary to the olivine-pigeonite residues represented by the majority of monomict ureilites.The most abundant population is characterized by albitic plagioclase in association with pyroxenes, phosphates, ilmenite, silica, and incompatible-element enriched glass. Model calculations suggest that it formed by extensive fractional crystallization of the earliest melt(s) of precursor materials from which the most magnesian (shallowest) olivine-pigeonite ureilites formed. A less abundant population, characterized by labradoritic plagioclase, may have formed from melts complementary to more ferroan olivine-pigeonite ureilites. and derived from deeper in the UPB. The third population, characterized by the presence of olivine and augite, could only have formed from melts produced at greater depths in the UPB than the olivine-pigeonite ureilites. Many other feldspathic clasts cannot be positively associated with any of these three populations, because their mafic mineral compositions exhibit carbon redox control. However, they may be products of early crystallization of basaltic melts produced on the UPB, before carbon was exhausted by reduction.Partial melting on the ureilite parent body was a fractional (or incremental) process. Melts were produced early in UPB history, and most likely extracted rapidly, thus preserving primitive chemical and oxygen isotopic signatures in the residues. Copyright (C) 2004 Elsevier Ltd.